In these days, automobiles are equipped with radio communication devices including GPS (Global Positioning System), VICS (Vehicle Information and Communication System), ETC (Electronic toll Collection system) and remote keyless entry units. Each of these radio communication devices has antennas and these antennas are brought together into one case as a composite device, which has been proposed in Japanese Laid open Patent publication No. 2001-267843.
Among the plural antennas integrated in this composite antenna, since GPS antennas need to receive electric wave coming from every direction, they are required to have predetermined sensitivity ranging from the vertical direction to the approximately horizontal direction. Since the VICS antennas need to receive electric wave coming from roadside transmission antennas while auto mobiles with the antennas mounted on passing from in front of the transmission antennas until past the transmission antennas, the antennas are required to have predetermined sensitivity from the front to the rear of the auto mobiles. On the other hand, as ETC antennas are required to satisfy the system requirements that when auto mobiles approaches in front of a toll booth the radio units of auto mobiles should start communication with radio units of the toll booth and communication should finish communication before the automobiles reach the toll booth, the ETC antennas often have a gain peak diagonally to the front of the vehicles.
However, as the plural antennas of such a composite antenna device are arranged on one circuit board closely to each other, electromagnetic coupling of the antennas affects their antenna characteristics (if plural antennas are close to each other, one antenna is affected even by another not-operating antenna), and accordingly it becomes difficult to adjust the antennas to have desirable antenna characteristics. Particularly, when the directivity of a certain antenna such as an ETC antenna is adjusted, such an adjustment becomes required that the circuit board is tilted, resulting in affecting the directivity of another antenna, and thereby making it difficult to achieve a composite antenna device having totally desired characteristics.
In addition, regarding characteristics other than directivity, such as Voltage Standing Wave Ratio (VSWR) and the axial ratio of circular polarization antenna, as the method of attachment to vehicle, the length of cables, influence from vehicle and the like are different depending on the type of the vehicle on which the antenna is mounted, it becomes required to adjust electrode dimensions of each antenna, the position of power feed point. However, if plural antennas are arranged close to each other on one circuit board, isolation is reduced and thereby it is difficult to adjust the characteristics of each of the antennas independently. For this reason, it is contemplated spacing the antennas so as to provide enough isolation. However, the antenna arranged on one board reduces the freedom degree and it becomes difficult to hold enough space between the antennas.
Further, if plural antennas are housed in one radome, the radome may be of inappropriate form for some of the antennas and the antennas characteristics such as circular polarization may be undesirable one. On the other hand, design of an appropriate radome suitable for all the antennas causes a problem of the cost.
Furthermore, in assembling a composite antenna device, as antenna devices are generally different in shape from other electronic devices, first the electronic devices are mounted on a circuit board by automated machine, and then, the antenna devices are mounted on the circuit board manually. On this account, the already mounted electronic devices other than the antenna devices are damaged by thermal and mechanical stress or static electrical charge applied to the circuit board in mounting the antenna devices thereon, thereby reducing a yield ratio or reliability.
Furthermore, as some users do not wish mounting of ETC devise, the composite antenna device is preferably configured not to have only the ETC antenna mounted on. In addition, as a vehicle with no ETC antenna implemented at first is sometimes retrofitted with an ETC antenna, a composite antenna device is preferably configured to be retrofitted with an ETC antenna.
However, a conventional antenna device is used to satisfy the above-mentioned requirements, there comes a need for preparing two types of circuit board, such as a circuit board for mounting an ETC antenna on and a circuit board for not mounting an ETC antenna. When an ETC antenna is retrofitted on the circuit board on which no ETC antenna has been mounted at first, the ETC antenna has to be mounted on the circuit board in the composite antenna device by soldering, which is actually difficult to be made by users or automobile dealers.
The present invention has an object to solve the above-mentioned problem to provide a composite antenna device which has antennas of respective appropriate characteristics and is allowed to flexibly respond to difference in vehicle type and change in antenna configuration.